• Geophysics and Geophysical Exploration
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• v.5 no.4
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• pp.299-308
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• 2002

We have derived an analytical expression for the sensitivity of the frequency domain small-loop electromagnetic (EM) surveys over a two-layer earth in order to estimate the depth of investigation with an instrument having the source-receiver separation of about 2 m. We analyzed the sensitivities to the lower layer normalized by those to the upper half-space and estimated the depth of investigation from the sensitivity analyses and the mutual impedance ratio. The computational results showed that the in-phase components of the sensitivity to the lower layer dominates those to the upper layer when the thickness of the upper layer is less than 20 m, while the quadrature components are not sensitive to the lower layer over the entire frequency range. Hence we confirmed that the accurate measurement of the in-phase component is essential to increase the depth of investigation in the multi-frequency small-loop EM survey. When conductive basement of 10 ohm-m underlies the upper layer of 100 ohm-m, an accurate measurement of the in-phase components ensures the depth of the investigation more than 10 m even accounting a noise effect, from which we conclude that the small-loop EM survey is quite effective in imaging the conductive plume down to a considerable depth. On the other hand, in the presence of the resistive basement of 1,000 ohm-m, the depth of investigation may not exceed 5 m considering the instrumental accuracy, which implies that the application of the small-loop EM survey is not recommended over the resistive environment other than detecting the buried conductor.

• Geophysics and Geophysical Exploration
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• v.7 no.2
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• pp.117-120
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• 2004

Immersive virtual reality provides an effective way of visualizing and analyzing various spatial data, such as wireline logs, three-dimensional seismic, and interpreted geologic boundaries, and etc. Although it is a valuable tool for oil and gas exploration, its usage has been limited to a specific area because of its high development costs. This paper describes the development of an immersive virtual reality system, known as CAVE (Cave Automatic Virtual Environment) that maximizes immersiveness with reasonable prices by using general purpose PC and projectors.

• Geophysics and Geophysical Exploration
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• v.7 no.1
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• pp.33-40
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• 2004

Numerical and laboratory studies have been conducted to test the ability of Electrical Resistance Tomography-a technique used to map the electrical resistivity of the subsurface-to delineate contaminant plumes. Two-dimensional numerical models were created to investigate survey design and resolution. Optimal survey design consisted of both downhole and surface electrode sites. Resolution models revealed that while the bulk fluid flow could be outlined, small-scale fingering effects could not be delineated. Laboratory experiments were conducted in a narrow glass tank to validate theoretical models. A visual comparison of fluid flow with ERT images also showed that, while the bulk fluid flow could be seen in most instances, fine-scale effects were indeterminate.

• Journal of the Korean Geophysical Society
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• v.5 no.3
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• pp.175-198
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• 2002

The techniques using geophysical methods were adopted to obtain quantitative criteria for assessment of grouting effectiveness. Various surface and borehole geophysical surveys including seismic, GPR(ground penetrating radar), resistivity and electromagnetic methods were conducted in fractured rock pilot site before and after grouting execution. However, it is not enough that geophysical data provide criteria for field engineers to confirm the grouting effectiveness in that site even though there is somewhat difference before and after grouting. This study will be continued for the detailed criteria and assessment of grouting effectiveness in other sites.

• Journal of the Korean Geophysical Society
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• v.4 no.1
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• pp.1-10
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• 2001

A number of tangible cultural properties have been left to suffering damage without any scientific conservation or maintenance. We conducted nondestructive geophysical explorations around the Western pagoda of the Iksan Mireuksa Temple for the purpose of preparing the counterplan of its conservation and maintenance and of utilizing the geophysical information for the design of repair. Geophysical image of the shallow subsurface around the construct resulting from electric resistivity, seismic refraction, and GPR methods carried out along 6 lines in the site was used to investigate the relationship between the foundation characteristics and the structural safety. Tilting of the pagoda southwest towards seems to result from the low resistivity zones found in the southwestern part. The GPR and seismic surveys revealed a boundary at depth of 3.3～3.5m dividing into two layers, compacted overlaid soil and the original ground. The boundary appears to dip southwest. The artificial layer as a foundation does not covers as much as the bottom area of the pagoda. This top soil dipping southwest seems to result in tilting of foundation southwestward towards. Our geophysical result suggests ground reinforcement in the western part of the survey area for the conservation of the construct.

• Geophysics and Geophysical Exploration
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• v.23 no.3
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• pp.141-148
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• 2020

A drone platform built by Korea Institute of Geoscience and Mineral Resources (KIGAM) is introduced. The platform consists of various drone systems developed at KIGAM for photogrammetry, remote exploration, physical exploration, field operation methods, a vehicle-based drone control center, as well as a drone data platform for data storage, sharing, analysis, and visualization of the acquired data. The performance of the drone platform is verified using results obtained with the various systems, which are tested individually and in various combined applications. Finally, the possibility of using the KIGAM drone platform for geological surveys and mineral resource exploration is discussed.

• Geophysics and Geophysical Exploration
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• v.14 no.2
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• pp.152-157
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• 2011

The small-loop electromagnetic (EM) method is one of the rapid and non-destructive geophysical methods and has been used widely for many geophysical investigations, particularly for shallow engineering and environmental surveys. Especially in the shallow marine environment, the small-loop EM technique is very effective because of rapid and convenient data acquisition, large signal and low noise level. However, the method has been rarely applied in the very conductive marine environment since it's penetration or investigation depth might be considered too low. In this study, we demonstrated that the small-loop EM method can be effectively applied in the extremely conductive marine environment through the analysis of 1D small-loop EM data. Furthermore, we confirmed that the resistivity distribution under the sea bottom can be quantitatively predicted from the 1D inversion results of synthetic and field data.

• Journal of the Korean Geophysical Society
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• v.7 no.3
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• pp.171-183
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• 2004

Electrical resistivity survey is widely used to investigate the stability of center-core type fill dam against the seepage phenomenon. In this study, we analyze the resistivity information obtained on a earth fill dam and compare it with the geotechnical SPT result. The analysis shows that the zones showing low resistivity value generally have low N value. However, some zones with high resistivity pattern do not accompany the increase of N value, and even showing low N value. These results imply that the direct identification of resistivity value to the real status of the core material of fill dam is impossible, and a highly resistive zone may be in serious status due to the effect increasing the resistivity value by the piping condition. Therefore, multiple exploration should be planned to reduce the uncertainty in application of geophysical methods to dam safety evaluation.

• Geophysics and Geophysical Exploration
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• v.8 no.2
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• pp.119-128
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• 2005

The assessment of erosional and depositional patterns near bridge piers is essential to understand the fluvial scour process. Geophysical surveys are particularly effective in determining the riverbed variations in a river and may also be of value for obtaining the previous scour history below the riverbed profile. In this study, GPR (Ground Penetrating Radar), as a non-destructive geophysical technique, was used to assess the existence and depth of existing and infilled scour thickness, streambed materials, and pre- and post- scour surfaces at the bridge piers in Han River, June 2002 and October 2002. The GPR acquisition system used for obtaining profiles of the shallow subsurface deposits was a portable GSSI SIR 2000 system with 100 and 400 MHz antennas. The GPR data obtained along the 24 bridge piers in the flow direction of the river and in the surroundings of 5 bridge piers were compared and presented in this study. It is concluded that GPR surveys can be effective in determining both the water depth and sub-bottom geological structure near the bridge piers and abutments provided that the appropriate instrumentation and operational procedures are applied.

• Geophysics and Geophysical Exploration
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• v.7 no.1
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• pp.93-98
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• 2004

The sand-covered Muweilah archaeological site in the United Arab Emirates (UAE) is a unique Iron Age site, and has been subject to intensive investigations. However, excavations are time consuming and may require twenty years to complete. Thus geophysical surveys were undertaken with the objective of characterising the site more expeditiously. This paper presents preliminary results of these surveys. Ground penetrating radar (GPR) was tested as a primary imaging tool, with an ancillary shallow time domain EM (MetalMapper) system. Dense 3D GPR datasets were migrated to produce horizontal (plan view) depth slices at 10 cm intervals, which is conceptually similar to the archaeologists' excavation methodology. The objective was to map all features associated with anthropogenic activity. This required delineating extensive linear and planar features, which could represent infrastructure. The correlation between these and isolated point reflectors, which could indicate anthropogenic activity, was then assessed. Finally, MetalMapper images were used to discriminate between metallic and non-metallic scatterers. The moderately resistive sand cover allowed GPR depth penetration of up to 5 m with a 500 MHz system. GPR successfully mapped floor levels, walls, and isolated anthropogenic activity, but crumbling walls were difficult to track in some cases. From this study, two possible courtyard areas were recognised. The MetalMapper was less successful because of its limited depth penetration of 50 cm. Despite this, the system was still useful in detecting modem-day ferruginous waste and bronze artefacts. The results (subject to ongoing ground-truthing) indicated that GPR was optimal for sites like Muweilah, which are buried under a few metres of sand. The 3D survey methodology proved essential to achieve line-to-line correlation for tracking walls. In performing the surveys, a significant improvement in data quality ensued when survey areas were flattened and de-vegetated. Although MetalMapper surveys were not as useful, they certainly indicated the value of including other geophysical data to constrain interpretation of complex GPR features.